Freezing level

Freezing level: height where air cools to 0 °C.

• Above it, air is below 0 °C: airframe icing possible in cloud or precipitation.
• Icing needs air at or below 0 °C and visible moisture (TC AIM AIR 2.12.3.1).

Calculate (ISA lapse rate 1.98 °C per 1000 ft, TC AIM AIR 1.5.1):

freezing level (ft AMSL) = elevation + (surface temp ÷ 1.98) × 1000

Forecast:

• GFA icing/turbulence chart gives actual freezing level directly (TC AIM MET 4.12).
• Aviation forecasts abbreviate it FZLVL (TC AIM MET Table 15.1).

Unit trap:

• Freezing: 0 °C = 32 °F.
• Surface temp in °F: convert to °C first.
• Elevation in metres: ×3.28 for feet.

Trap: answer is height above sea level: add station elevation.

Practice